Vibrational spectroscopies provide important information concerning the structure, composition, and orientation of molecules. Infrared spectroscopy may be used to determine molecular composition and structure, where the “chemical fingerprints” of functional groups may be obtained by directly exciting dipole-active molecular vibrations with resonant infrared light.
Surface-enhanced strategies have utilized metallic substrates, such as roughened or metal island films or deposited nanoparticles, where molecules are located on or near the structure. The structures may provide an intense, local field enhancement when illuminated, and convey the resulting molecular response effectively to the far field, where it may be detected. The enhancement of vibrational modes is believed to scale as |E|2 of the local field.
The local fields at illuminated metal structures responsible for surface-enhancements are due to excitation of the collective oscillations of the metal electrons of the structure, known as surface plasmons. The size, shape and composition of the metal structure determine the structures resonant frequencies. If a metallic antenna structure has a plasmon resonance at the same frequency as a molecular vibration, the metal and molecule systems may couple, resulting in spectral features with Fano lineshapes characteristic of a coupling between broad and narrow energy states.
For Surface-Enhanced Infrared Absorption (SEIRA), simple antenna structures such as nanorods may provide enhanced IR vibrational signals. However, a high-intensity tunable light source, such as a synchrotron, may be required for adequate signal intensity. For conventional IR sources, large arrays of nanoscale antennas may be required to provide a sufficiently strong signal for detection.